Treatment of aluminum



United States Patent 3,468,766 TREATMENT OF ALUMINUM William C. Lang, Cypress, Califl, assignor, by mesne assignments, to McDonnell Douglas Corporation, Santa Monica, Calif., a corporation of Maryland No Drawing. Filed Aug. 27, 1965, Ser. No. 483,360 Int. Cl. C23f 17/00; B32b /20 US. Cl. 204-35 16 Claims ABSTRACT OF THE DISCLOSURE A process of increasing the corrosion resistance of an aluminum surface by first anodizing in chromic acid; secondly coating the surface with a solution of an alkyl aryl polyethylene glycol such as octylphenoxy poly(ethyleneoxy) ethanol or nonylphenoxy poly(ethyleneoxy) ethanol; and thirdly treating the coated surface with a solution of a chromate.

This invention relates to the sealing of an anodized aluminum surface to provide protection against corrosion of such surface, and is particularly concerned with novel procedure for provision of an improved seal on chromic acid anodized aluminum, to afford additional corrosion protection and other improved characteristics.

For many purposes, both ornamental and structural, aluminum surfaces are protected by an oxide coating produced on the surface by electrolytic oxidation of the aluminum by exposure thereof as an anode in an acid electrolyte such as an aqueous solution of sulfuric acid or chromic acid. This procedure is commonly known as anodizing and the resulting aluminum oxide surface which is thus produced is termed an anodized coating. To improve the corrosion resistance of such anodic coating, it is the usual practice to treat such coating in various Ways to provide a seal. One common method for providing such a seal is to treat the anodic coating in hot Water. However, such a seal for many applications such as for structural use of the aluminum, does not rovide sufficient corrosion resistance and such seal is often easily penetrated. Solutions of various substances have also been applied in the prior art to provide a seal for an anodized aluminum coating, but for one reason or another such seals have not proven satisfactory for providing anodized aluminum, and particularly chromic acid anodized aluminum employed for structural purposes in the aircraft industry, with the desired degree of corrosion resistance demanded for this important application.

The aircraft industry has traditionally employed chromic acid anodizing to provide corrosion protection for the aluminum alloys used in the construction of aircraft. However, low maintenance aircraft require maximum corrosion protection of the aluminum alloy used, and hence the provision of a suitable seal on such anodic coatings is of particular importance.

It is accordingly an object of the invention to provide improved procedure for sealing anodized aluminum coatings, to provide improved corrosion resistance thereof.

Another object is the provision of procedure for providing a seal on chromic acid anodized aluminum, to impart maximum corrosion resistance to such anodized aluminum.

Another object is to provide a process for sealing chromic acid anodized aluminum to afford maximum corrosion protection with minimum reduction of fatigue strength of the base aluminum.

Yet another object of the invention is to develop procedure which provides a seal on chromic acid anodized aluminum, to impart maximum corrosion resistance to such anodized aluminum and also providing corrosion ice protection to the aluminum where the anodic film has accidently been removed in localized areas.

A still further object of the invention is the development of a process which provides a seal on chromic acid anodized aluminum, to impart maximum corrosion resistance to such anodized aluminum while providing a surface which is receptive and compatible with forming a good bond of paints, organic sealants, and other coating materials to the anodized aluminum.

Other objects and advantages of the invention will appear hereinafter.

According to the invention, it has been found that the corrosion resistance of anodized aluminum, particularly chromic acid anodized aluminum, can be substantially improved by contacting said anodized aluminum, or an object formed therefrom, e.g., an aircraft part, with a solution of an alkyl aryl polyethylene glycol in deionized water, and then contacting the so-treated aluminum with an aqueous solution of a soluble chromate, particularly a soluble dichromate. It has also been found unexpectedly that treatment of a chromic acid anodized aluminum by the process of the invention improves the fatigue strength of the anodized aluminum, and that the resulting fatigue strength of such chromic acid anodized aluminum sealed according to the invention process is superior to the fatigue strength of sulfuric acid anodized aluminum treated according to the invention procedure.

Tests performed on chromic acid anodized aluminum treated in accordance with the sealing process of the invention have exhibited excellent salt spray resistance, resistance to corrosive chemicals such as acetic acid and sodium chloride, improved corrosion resistance in heat affected zones in the area of spot-welding, improved corrosion resistance of damaged or scratched areas of the anodic coating and provision of an excellent base for paints and other organic materials.

In processing aluminum objects or parts according to the invention, the aluminum objects, prior to anodizing, preferably are first cleaned and deoxidized in the usual manner employing conventional treating solutions. The objects are then anodized in the conventional manner in an aqueous chromic acid solution, with the aluminum objects or parts connected as anode in such operation. Thus, for example, such parts can be anodized in an aqueous chromic acid solution of from about 1 to about 10% chromic acid concentration at voltages ranging from about 20 to about volts, and for a period ranging from about 10 to about minutes, the chromic acid solution being maintained preferably at elevated temperature, e.g., of the order of about 85 to about F.

The chromic acid anodized aluminum objects are then rinsed to remove residual chromic acid, and the so rinsed aluminum objects are then treated as by immersion in a hot solution in deionized water of an alkyl aryl polyethylene glycol, In preferred practice, the glycol is an alkylphenoxy poly(ethyleneoxy) ethanol, said alkyl group being selected from the class consisting of octyl and nonyl, but most desirably is a nonylphenoxy poly (ethyleneoxy)ethanol. The number of ethylene groups present in the glycol component can vary. For example, the number of such ethylene groups can range from about 6 to about 20, and is preferably about 9 to about 10. Thus, a class of suitable polyalkylene glycol components can be represented by the formula where R is alkyl, preferably octyl or nonyl, and n is an integer of from 6 to 20, preferably 9 to 10.

Treatment of the anodized aluminum in such deionized solution of the above polyethylene glycol is generally carried out at a solution temperature of about 175 F. up to about boiling, usually in the range from about 185 to about 212 F. The concentration of the glycol component in the solution can vary but is generally in the range from about 0.5 to about 10 ml. per liter of solution, and is preferably in the range from about 0.5 to about 7.5 ml. per liter. The pH of the solution is usually maintained between about 4 and about 8, and preferably is in the range of about 4 to about 6.5. The time of treatment of the aluminum objects in the aqueous glycol solution can range from as little as 3 minutes up to about 80 minutes or more. However, in preferred practice, time of treatment is usually maintained between about 5 and about 20 minutes. Although increasing the time of treatment in the glycol solution 'was found to improve the corrosion resistance of the aluminum, increasing such period beyond about 20 minutes was also found to leave a deposit of the alkyl aryl polyethylene glycol component on the surface of the anodized aluminum, which was found difficult to remove by rinsing in water.

The most important processing parameter with respect to this operation of treatment in the aqueous glycol solution, is the necessity for the use of substantially pure water in the solution. Thus, deionized water is employed in initially making up the solution of the above noted glycol component. However, during treatment of the anodized aluminum objects in the solution, the aqueous solution of the glycol component becomes contaminated and to achieve best results according to the invention, it has been found most desirable to maintain the water in a highly purified state by continuous circulation of the aqueous solution of the glycol component through a water purification system. It has been found that particularly good results are achieved by continuous circulation of the aqueous solution of the alkyl aryl polyethylene glycol through ion exchange resins to remove contaminants such as silicates and other contaminants which may be present in the solution or introduced therein during the processing of the chromic acid anodized aluminum parts. Both anionic and cationic exchange resins can be employed for this purpose, such as, for example, the materials marketed as lllco A-244 anionic exchange resin and Illco C-2l1 polystyrene cationic exchange resin.

The deionized water employed in the aqueous solution of the glycol component, as previously noted, should be substantially free of contaminants such as silicates and carbonates. For this purpose, the initially deionized water, and the water in the solutions containing glycol component passed through the ion exchange resins and recirculated for treatment of the anodized aluminum parts, for best results should have an electrical resistance of at least about 50,000 ohms, generally between about 50,000 and about 100,000 ohms. If desired, it will be understood that means other than ion exchange resins can be employed for maintaining the above noted desired purity of the water in the aqueous solution of the glycol component.

The so-treated aluminum objects are then rinsed to remove excess alkyl aryl polyethylene glycol, and are then immersed in a hot aqueous solution of a water soluble chromate. Although any water soluble chromate salt such as an alkali metal chromate can be used, preferably, such chromate is a water soluble dichromate such as ammonium, or an alkali metal, e.g., sodium or potassium, dichromate. In preferred practice, sodium dichromate or potassium dichromate is employed. The concentration of chromate component, e.g., dichromate, in said solution can be varied but is generally in the range of about 0.5 to about by weight of the solution, preferably about 2.5 to about 7.5% by weight of this solution. It has been found for best results that the pH of the solution should be maintained between about 3.0 and about 6.5 and in preferred practice is maintained between about 3.3 and about 5.5. It has been found that maintenance of the pH of the dichromate solution Within the ranges noted above provides not only good corrosion protection, but good adhesion of the sealant coating to the anodized aluminum, and also good paint adhesion, where a paint coating is subsequently applied over the sealed anodic coating.

The aqueous chromate or dichromate solution is usually maintained at a temperature in the range of about F. to about boiling during the processing of the anodized aluminum objects therein, and preferably ismaintained in the range of about to about 212 F. Time of treatment in the aqueous chromate or dichromate solution can range from about 3 to about 30 minutes, and is usually in the range of about 5 to about 15 minutes. Following treatment of the anodized aluminum objects in the aqueous chromate or dichromate solution, the so-treated and sealed anodized aluminum parts or objects are then rinsed to remove excess dichromate.

The term aluminum employed herein is intended to denote either pure aluminum or any of the alloys thereof, including, for example, 2024 aluminum, 7075 aluminum, 6061 aluminum, and the like.

The following are examples of practice of the invention.

Example 1 Nonclad 7075T6 aluminum test panels each 3" X 5 /2 and 0.040" thick are cleaned in a conventional aluminum cleaner and deoxidized in a deoxidizer of a conventional type.

The panels are then water rinsed and are anodized at 40 volts for about 43 minutes in a 5% aqueous chromic acid solution at a temperature of about 95 F.

The test panels are then rinsed and immersed in a solution of 5 ml. per liter of nonylphenoxy poly(ethyleneoxy) ethanol, and containing about 10 ethylene groups, marketed as Igepal CO-630, in deionized water having a resistance of about 100,000 ohms. The pH of the solution is adjusted to between about 5.0 and about 6.0 by the addition of a few drops of sodium hydroxide per liter of solution, and the temperature of the solution maintained at about 205 to about 210 F. The time of treatment of the panels in this solution is about 15 minutes.

During the period of treatment of the panels in the above noted deionized water solution of the polyethylene glycol noted above, such solution is circulated in series through anionic and cationic exchange resins, marketed as Illco A-244 anionic exchange resin and Illco C-21l polystyrene cationic exchange resin to maintain the purity of the solution corresponding to a resistance thereof ranging from about 50,000 to about 100,000 ohms.

The test panels are then rinsed and respective panels are then sealed in different sodium dichromate aqueous solutions containing a concentration of 5% by Weight of sodium dichromate, the pH of such solutions ranging from about 3.3 to about 6.0, the desired pH of such solutions being obtained by addition of chromic acid 01- sodium hydroxide, as required. The solutions are maintained at a temperature of about 210 to 212 F., and the samples are treated in the solutions for a period of about 15 minutes. Following this treatment, the panels are removed from the aqueous dichromate solutions and rinsed.

The test panels are subjected to salt spray testing in a salt spray cabinet in accordance with standard testing procedure set forth in Federal Method Standard 151A, Method 811.1A. The panels are subjected to exposure in the salt spray cabinet for a period of 88 hours. Following salt spray exposure the test panels are removed from the cabinet, rinsed in tap water and air dried. They are then evaluated visually for corrosion, pits, bloom and general deterioration of the anodic film.

All of the test panels so treated are observed either to have no corrosion or stains, or to have only very slight corrosion or staining in areas where a scribe is used to damage the coating prior to salt spray exposure.

Certain of the panels tested as noted above are painted on one side with a paint primer having fluid resistant properties, prior to salt spray exposure, and then such panels are air dried prior to introduction in the salt spray cabinet. Following exposure to salt spray for the above noted period of 888 hours, adhesion testing of the primer coating shows good adhesion of such coating to the previously sealed anodized aluminum test panels.

Example 2 The procedure of Example 1 is repeated, except that the polyethylene glycol component employed in the treating solution containing deionized water, is octylphenoxy poly(ethyleneoxy) ethanol.

Results obtained are similar to those obtained in Example 1.

Example 3 The procedure of Example 1 is carried out, except that the water employed in the aqueous solution of the polyethylene glycol component is tap water.

Following salt spray exposure testing according to the procedure of Example 1 above, the anodic coating of the aluminum test panels is observed to contain excessive pits, corrosion, bloom, and stains.

Example 4 Four series of aluminum test panels in the form of non-clad 7075T6 aluminum and of a size 3" x 5" x .040" thick are treated according to the procedure noted below.

The first series of panels are anodized in aqueous sulfuric acid in the conventional manner employing about 16 to about 18% sulfuric acid concentration, operating temperatures of about 68 to 70 F., and voltages of about 18 to about 22 volts. Following the anodizing operation this series of panels is treated in a sodium dichromate solution containing about 6 percent by weight of sodium dichromate.

The second series of aluminum test samples are anodized in a chromic acid solution of about 5 percent chromic acid concentration and at about 40 to 42 volts, at temperature of about 95 F. The chromic acid anodized aluminum samples are rinsed but are not further treated and remain unsealed.

The third series of samples are first anodized in a 5 percent aqueous chromic acid solution at reduced voltage of about 20 to-about 21 volts and at elevated temperature of the order of about 95 F. The so-anodized samples are then subjected to treatment in deionized water for sealing the anodic coating in the usual manner.

The four series of samples are treated according to the procedure of Example 1, first by application of a chromic acid anodic coating, and then by treatment of the soanodized samples first in the polyethylene glycol solution in deionized water, followed by treatment in the sodium dichromate solution.

Each of the above four series of aluminum samples are anodized to approximately the same thickness of anodic oxide coating.

All four series of aluminum samples treated as described above are then tested in tension-tension fatigue under axial loading on a Sontag fatigue machine operating at 1800 cycles per minute. The broken ends of the test specimens are protected with a stop oif lacquer masking material and then the specimens are placed in a salt spray test cabinet and tested for corrosion resistance therein for a period of time of about 6000 hours.

The results of these tests are set forth briefly below.

From the test results noted above, it is seen that chromic acid anodized aluminum which is subjected to the dual sealing procedure of the invention employing a first solution of the polyethylene glycol material in deionized water, followed by treatment in the solution of a dichromate, has both good corrosion resistance and good fatigue life where as the sulfuric acid and chromic acid anodized samples in series 1, 2, and 3, whether unsealed, or sealed by other procedures, is deficient either in corrosion resistance or fatigue life, and do not have good combined characteristics for both these parameters.

From the foregoing, it is seen that the invention provides a novel dual sealing procedure for sealing chromic acid anodized aluminum to substantially improve the corrosion resistance of the anodic coating, and at the same time afford minimum reduction of the fatigue strength of the base aluminum, particularly as compared to sulfuric acid anodized aluminum.

While I have described particular embodiments of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention, and within the scope of the appended claims.

I claim:

1. The method of increasing the corrosion resistance of a chromic acid anodized aluminum object, which comprises contacting said object with a solution of an alkyl aryl polyethylene glycol containing about 6 to about 20 ethylene groups in deionized water, and then contacting said object with an aqueous solution of a soluble chromate.

2. The method of increasing the corrosion resistance of a chromic acid anodized aluminum object, which comprises contacting said object with a solution in deionized water of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, said alkyl group being selected from the class consisting of octyl and nonyl, and then contacting said object with an aqueous solution of a soluble chromate.

3. The method of increasing the corrosion resistance of a chromic acid anodized aluminum object, which comprises contacting said object with a solution in deionized water of a nonylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, and then contacting said object with an aqueous solution of a soluble dichromate.

4. The method of increasing the corrosion resistance and fatigue strength of a chromic acid anodized aluminum object, which comprises contacting said object with a hot solution in deionized water of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, said alkyl group being selected from the class consisting of octyl and nonyl, and then contacting said object with a hot solution of a soluble dichromate having a pH in the range from about 3.0 to about 6.5.

5. The method of increasing the corrosion resistance and fatigue strength of a chromic acid anodized aluminum object, which comprises contacting said object with a hot solution in deionized water of nonylphenoxy poly (ethyleneoxy) ethanol containing about 9 to about 10 ethylene groups, and then contacting said object with a hot solution of an alkali metal dichromate having a pH in the range from about 3.0 to about 6.5.

6. The method of increasing the corrosion resistance of a chromic acid anodized aluminum object, which comprises contacting said object with a solution of an alkyl aryl polyethylene glycol containing about 6 to about 20 ethylene groups in deionized Water, maintaining the purity of said deionized water such that the electrical resistance thereof is at least about 50,000 ohms, and then contacting said object with an aqueous solution of a soluble chromate.

7. The method of increasing the corrosion resistance of a chromic acid anodized aluminum object, which comprises contacting said object with a solution in deionized water of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, said alkyl group being selected from the class consisting of octyl and nonyl, maintaining the purity of said deionized water during treatment of said aluminum object such that the electrical resistance of said solution is in the range of about 50,000 to about 100,000 ohms, and then contacting said object with an aqueous solution of a soluble dichromate.

8. The method of increasing the corrosion resistance and fatigue strength of a chromic acid anodized aluminum object, which comprises contacting said object with a hot solution in deionized water of about 0.5 to about 10 ml. per liter of solution of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about ethylene groups, said alkyl group being selected from the class consisting of octyl and nonyl, and then contacting said object with a hot solution of about 0.5 to about 10% by weight of a soluble dichromate having a pH in the range from about 3.0 to about 6.5.

9. The method as defined in claim 8, the temperature of each of said solutions being in the range of about 175 F. to about boiling.

10. The method of increasing the corrosion resistance and fatigue strength of a chromic acid anodized aluminum object which comprises contacting said object with a solution in deionized water of about 0.5 to about 10 ml. per liter of solution, of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, said alkyl groups being selected from the class consisting of octyl and nonyl, said solution having a pH in the range from about 4.0 to about 8.0, at a temperature in the range of about 175 F. to about boiling, and then contacting said object with an aqueous solution of about 0.5 to about 10% by weight of an alkali metal dichromate, at a temperature in the range of about 175 F. to about boiling, said dichromate solution having a pH in the range from about 3.0 to about 6.5.

11. The method as defined in claim 10, wherein said ethanol is a nonylphenoxy poly (ethyleneoxy) ethanol.

12. The method of increasing the corrosion resistance and fatigue strength of a chromic acid anodized aluminum object which comprises contacting said object for a period of about 3 to about 80 minutes with a solution in deionized water of about 0.5 to about 7.5 ml. per liter of solution of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, said alkyl group being selected from the class consisting of octyl and nonyl, said solution having a pH in the range of about 4.0 to about 6.5, at a temperature in the range of about 185 to about 212 F., the water employed in said solution having a resistance of the order of at least about 50,000 ohms, and then contacting said object for a period of about 3 to about 30 minutes with an aqueous solution of about 2.5 to about 7.5% by weight of an alkali metal dichromate, at a temperature in the range of about 180 to about 212 F., said dichromate solution having a pH in the range from about 3.3 to about 5.5.

13. The method of increasing the corrosion resistance and fatigue strength of a chromic acid anodized aluminum object which comprises contacting said object for a period of abput 5 to about 20 minutes with a solution in deionized water of about 0.5 to about 7.5 ml. per liter of solution of p-nonylphenoxy poly (ethyleneoxy) ethanol containing about 9 to about 10 ethylene groups and having a pH in the range of about 4.0 to about 6.5, at a temperature in the range of about 185 to about 212 F., the water employed in said solution having a resistance in the range of about 50,000 ohms to about 100,000 ohms, and said solution being substantially free of silicates and carbonates, and maintaining the above noted purity of said solution during treatment therein of said aluminum object, and then contacting said object for a period of about 5 to about 15 minutes with an aqueous solution of about 2.5 to about 7.5% by Weight of sodium dichromate, at a temperature in the range of about 180 to about 212 F., said dichromate solution having a pH in the range from about 3.3 to about 5.5.

14. The method as defined in claim 13, wherein the purity of said solution containing said ethanol is maintained by circulating said solution through an ion exchange resin.

15. A method for treatment of aluminum to increase its corrosion resistance, which comprises anodizing said aluminum in an aqueous chromic acid solution, contacting said anodized aluminum with a solution of an alkyl aryl polyethylene glycol containing about 6 to about 20 ethylene groups in deionized water, and then contacting said anodized aluminum with an aqueous solution of a soluble chromate.

16. A method for increasing the corrosion resistance and fatigue strength of an aluminum object, which comprises anodizing said aluminum object in an aqueous chromic acid solution, contacting said object with a solution in deionized Water of about 0.5 to about 10 ml. per liter of solution, of an alkylphenoxy poly (ethyleneoxy) ethanol containing about 6 to about 20 ethylene groups, said alkyl group being selected from the class consisting of octyl and nonyl, said solution having a pH in the range from about 4.0 to about 8.0, at a temperature in the range of about F. to about boiling, and then contacting said object with an aqueous solution of about 0.5 to about 10% by weight of an alkali metal dichromate, at a temperature in the range of about 175 F. to about boiling, said dichromate solution having a pH in the range from about 3.0 to about 6.5.

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